Non-linear video signal processors are known wherein a video signal is subjected to non-linear amplification within selected portions of the video signal range for providing improved contrast in displayed images.
O. H. Shade, for example, describes a non-linear video processor featuring both so-called "black stretch" and "white stretch" processing for improving detail in the darker and lighter areas of displayed images in U.S. Pat. No. 2,760,008 which issued Aug. 21, 1956. "Black stretch" is a term used to describe the non-linear processing of the luminance portion of a video signal such that the luminance signal gain is increased for middle valued luminance levels thereby providing improved contrast. In a similar manner, improved detail within lighter areas of displayed images is provided by increasing the luminance signal gain for video signals of relatively high level to "stretch", in a manner of speaking, the signal range of bright portions of displayed images. An advantage of the Shade system is that there is great flexibility in the choice of video signal transition points or "break points" as well flexibility in controlling video signal gain levels between the break points of the overall transfer function.
Although the Shade system provides improvement in the control of detail for dark scenes and light scenes, there are a relative large number of manual controls which require adjustment to realize these advantages. These include controls for gain setting and controls for threshold or "break point" setting. Proper adjustment of the overall system can be relatively complex due to the numerous manual controls involved.
More recently, integrated circuits have become available providing non-linear picture enhancement functions which feature some measure of automatic control so as to relieve the user of the burden of adjusting a number of individual controls. An example of such an integrated circuit is the type CX20125 "dynamic picture processing" integrated circuit manufactured by Sony Corp. This integrated circuit provides "black stretch" processing for improving contrast of lower level video signals. Briefly, after stripping sync from the video signal, the deepest black level is detected by a black peak hold circuit and it becomes the control signal of a gain control circuit and forms a feedback loop to perform black expansion automatically as a function of detected black peaks of the video signal.
Another approach to automatic control of non-linear processing for improved contrast in brighter picture areas is based upon analysis of the so-called "average picture level" (APL) of displayed images. Examples of "white stretch" processors, which improve detail in brighter picture areas are described, for example, by Lagoni In U.S. Pat. No. 5,003,394 entitled DYNAMIC VIDEO SYSTEM INCLUDING AUTOMATIC CONTRAST AND "WHITE STRETCH" PROCESSING SECTIONS which issued Mar. 26, 1991. In an embodiment of the Lagoni system white stretch processing is performed as a function of the average picture level (APL) of displayed images and contrast control is effected both by a user control signal source and a peak white signal detector in response to displayed images.
Other examples of non-linear processing are provided, for example, by Bell and Lagoni, in U.S. patent application Ser. No. 808327 filed Dec. 16, 1991 and entitled NON-LINEAR LUMINANCE SIGNAL PROCESSOR RESPONSIVE TO AVERAGE PICTURE LEVEL (APL) OF DISPLAYED IMAGE. A further example is described by Lagoni and O'Brien in U.S. patent application Ser. No. 808328 filed Dec. 16, 1991 and entitled VIDEO SYSTEM WITH FEEDBACK CONTROLLED "WHITE STRETCH" PROCESSING AND BRIGHTNESS CONTROL. In such systems, improved contrast in bright areas of a picture are provided by controlling the luminance signal gain in such areas as a function of the average picture level (APL) of the images that are displayed.